The current trend in communication satellites appears to be increasingly toward the use of the 12/14 GHz and higher frequency bands and the use of digital modulation formats with Time Division Multiple Access (TDMA) techniques. The former provides freedom from existing 4/6 GHz terrestrial interference and also provides higher antenna gain and narrower beams for a given size aperture, while digital transmission in conjunction with TDMA provides for more efficient utilization of the available satellite system resources.
A major drawback associated with 12/14 GHz and higher frequency systems is the signal attenuation associated with rainfall. In general, attenuation at these higher frequencies is an increasing function of rain rate, with the result that techniques must be employed to prevent excessive outage in areas where rain fades can occur.
Techniques which have been used to provide appropriate rain margin include (1) transmitting a burst several times, (2) increasing the radiated power of the satellite and earth stations, (3) improving the noise figure of the receivers, (4) installing larger ground station antennas, and (5) providing site diversity. In a typical site diversity arrangement, a main station serving a designated service area normally handles all communications with a satellite and is connected by a land communication link to a diversity station perhaps 10 or more kilometers away which is beyond the extent of a normal rain area. When the main station experiences a rain fade, a control circuit causes all communication with the satellite to be routed via the communication link and the diversity station. In this regard see, for example, U.S. Pats. No. 3,829,777 issued to T. Muratani et al on Aug. 13, 1974; U.S. Pat. No. 3,896,382 issued to B. Magenheim on July 22, 1975; U.S. Pat. No. 4,052,670 issued to T. Watanabe et al on Oct. 4, 1977; and U.S. Pat. No. 4,099,121 issued to K. J. F. Fang on July 4, 1978.
The use of diversity stations significantly increases the amount of equipment required and requires high-capacity links between the two diversity stations. The problem remaining in the prior art is to provide a diversity arrangement wherein the extra equipment required is significantly reduced.